Background: High-grade gliomas are among the leading cause of cancer-related death in children, with diffuse intrinsic pontine glioma (DIPG) being a highly lethal subtype. DIPG commonly exhibits a mutation in histone H3, referred to as H3K27M, which leads to gliomagenesis. However, the cell signaling events that promote DIPG growth are not fully understood. We have discovered and recently shown that DIPGs are reliant on extracellular signal-regulated kinase 5 (ERK5) for proliferation and survival. However, several critical knowledge gaps of ERK5 function in DIPG remain. We will use an innovative syngeneic DIPG model in combination with well-established models of DIPG to address several questions in this proposal. Specifically, we hypothesize that ERK5 promotes growth through both glioma intrinsic effects and pro-glioma activities of select glioma associated microglia and macrophages (GAMs). Significance: To support our preliminary data and test our hypothesis we will study the role of ERK5 in three aims. Aim 1: Revealing functions of ERK5 in GAMs and the importance of paracrine signaling with DIPG cells will provide valuable knowledge potentially applicable to the development of new therapeutic modalities. Aim 2: Building a new paradigm by understanding the dynamic characteristics of targeting multiple ERK pathways. Aim 3: ERK5 contains both a transactivation domain and kinase domain unlike all other ERKs. Here we will delineate the functional role of these domains and their contribution to glioma growth. Impact. This project leverages unique in vivo and in vitro model systems to study a highly aggressive brain tumor. Our research will inform molecular mechanisms and translational relevance of ERK5 in brain tumors, and be broadly applicable to other types of cancer and neurosciences.